Monolithic fuel cell having improved interconnect layer

ABSTRACT

A fuel cell comprises an anode, a cathode, a solid electrolyte disposed between the anode and the cathode, and interconnect means for providing an electrical connection located adjacent to either the cathode or the anode but not adjacent to the solid electrolyte. The interconnect means includes an interconnect layer having a composition of (i) a mixture of an electrical conductor and a ceramic matrix material that is sinterable in an oxidizing atmosphere at a temperature of less than about 1500° C., (ii) a mixture of a lanthanum chromite-based ceramic and a yttrium chromite-based ceramic, or (iii) a yttrium chromite-based ceramic of the form Y w-x-y  Ca x  Zr y  Cr v-z  Zn z  O 3  , where w is from about 0.9 to about 1.1, x is from about 0.1 to about 0.3, y is from about 0.001 to about 0.1, z is from about 0.1 to about 0.3, and v is from about 1 to about 1.2.

BACKGROUND OF THE INVENTION

This invention relates to fuel cells, and, more particularly, to theinterconnect layer of a monolithic solid oxide fuel cell.

A fuel cell is a device in which a fuel is electrochemically reactedwith an oxidant to produce a DC electrical output. A fuel cell includesan anode that defines a flow passage for the fuel, such as hydrogen or ahydrocarbon, and a cathode that defines a flow passage for the oxidant,such as air or oxygen. An electrolyte separates the anode and thecathode. In one type of fuel cell, the monolithic solid oxide fuel cell,the anode, the cathode, and the electrolyte are thin, layered corrugatedstructures, and the fuel cell further includes plenum conduits for theintroduction and removal of the fuel and oxidant.

Such fuel cells are described in more detail in U.S. Pat. Nos. 4,816,036and 4,913,982, whose disclosures are incorporated by reference. Briefly,in such a fuel cell the fuel flowing through the anode reacts with oxideions to produce electrons and water, which is removed in the fuel flowstream. The oxygen reacts with the electrons on the cathode surface toform oxide ions that diffuse through the electrolyte to the anode. Theelectrons flow from the anode through an external circuit and thence tothe cathode. The electrolyte is a nonconductor of electrons, ensuringthat they must pass through the external circuit to do useful work, butpermits the oxide ions to pass through from the cathode to the anode.

Each individual anode/electrolyte/cathode cell generates a relativelysmall voltage. To achieve higher voltages that are practically useful,the individual cells are connected together to form a battery. Themonolithic solid oxide fuel cell therefore contains an additional layer,an interconnect layer, between the cathode and the anode of adjacentcells.

The interconnect layer must be electrically conducting and must beformable at temperatures comparable with those required to form theother layers. In the technology described in the '036 and '982 patents,the monolithic solid oxide fuel cell is formed by a powder process,which includes sintering of the assembled structure, preferably in anoxidizing atmosphere. The sintering temperature dictated by thesintering requirements of the anode, the cathode, and the electrolyte istypically about 1400 C.-1500 C. The interconnect layer must besinterable at this same temperature, to a sufficiently good electricalconductivity and a relatively high density of at least about 94 percentof theoretical density, and without chemical interdiffusion with theneighboring layers.

The preferred interconnect material in the past has been magnesium-dopedlanthanum chromite. However, this material is not fully satisfactory insome circumstances because it may not sinter to a sufficiently highdensity at the sintering temperature of the anode, the cathode, and theelectrolyte. In order to sinter lanthanum chromite to high densities,firing temperatures greater than 1600 C. and an atmosphere having a lowoxygen partial pressure are required. These conditions, however, are notsatisfactory for other fuel cell components. At such high temperatures,diffusion and reaction between components becomes significant. Moreover,low oxygen partial pressures cause the decomposition of other materialsused in the fuel cell.

There is therefore a need for an improved interconnect approach for usein monolithic solid oxide fuel cells and related types of devices. Thepresent invention fulfills this need, and further provides relatedadvantages.

SUMMARY OF THE INVENTION

The present invention provides an approach for improving theinterconnect material of a fuel cell, and in particular a monolithicsolid oxide fuel cell, and for fuel cells prepared in accordance withthis approach. This approach achieves the required properties for theinterconnect material, including the high density of more than 94percent of the theoretical density, using the same sinteringtemperatures and conditions appropriate for the anode, the cathode, andthe electrolyte. The approach of the invention is fully compatible witheconomical manufacturing techniques such as those discussed in the '036and '982 patents.

In accordance with the present invention, a fuel cell includes an anode,a cathode, a solid electrolyte disposed between the anode and thecathode, and interconnect means for providing an electrical connectionlocated adjacent to the cathode or the anode but not adjacent to thesolid electrolyte. The interconnect means includes an interconnect layerhaving a composition that, in one form, includes a mixture of anelectrical conductor that provides the required electrical conductivityand a sinterable matrix that densifies in an oxidizing atmosphere at atemperature of less than about 1500 C. In another form, the interconnectlayer is formed of a mixture of a lanthanum chromite-based ceramic and ayttrium chromite-based ceramic. The interconnect layer may also be ayttrium chromite-based ceramic of the composition Y_(w-x-y) Ca_(x)Zr_(y) Cr_(v-z) Zn_(z) O₃, where w is from about 0.9 to about 1.1, x isfrom about 0.1 to about 0.3, y is from about 0.001 to about 0.1, z isfrom about 0.1 to about 0.3, and v is from about 1 to about 1.2.

The interconnect means may be provided as a first buffer layer having aregion with a composition that is intermediate between that of the anodeand that of the interconnect layer, a second buffer layer having aregion with a composition that is intermediate between that of thecathode and that of the interconnect layer, and the interconnect layerdisposed between the first buffer layer and the second buffer layer. Thebuffer layers may be either of constant composition or have a gradientcomposition variation between that of the interconnect layer and therespective anode or cathode layers. The buffer layers on either side ofthe interconnect layer permit the interconnect layer to sinter in anoxidizing atmosphere at a temperature of less than about 1500° C. Inthis aspect of the invention, the interconnect layer may be one of thosedescribed above, or another composition.

These approaches produce an interconnect means whose manufacture iscompatible with that of the other components of the fuel cell. Theresulting interconnect has a high density of more than 94 percent of thetheoretical density and functions to electrically connect adjacent cellsas required. Other features and advantages of the present invention willbe apparent from the following more detailed description of thepreferred embodiment, taken in conjunction with the accompanyingdrawings, which illustrate, by way of example, the principles of theinvention.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an elevational view of two adjacent electrochemical cells of amonolithic solid oxide fuel cell, connected in series through aninterconnect; and

FIG. 2 is an elevational view similar to that of FIG. 1, except that theinterconnect has an internally layered structure.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 1 illustrates a fuel cell 20 having two electrochemical cells 22.Each of the electrochemical cells 22 includes two electrodes, an anode24 and a cathode 26, with an electrolyte 28 between the anode 24 and thecathode 26. The anode 24, cathode 26, and electrolyte 28 are all in theform of layers of their respective materials of construction.Passageways 30, seen in cross section, run through the anode 24 so thatthe fuel for the fuel cell 20 may flow therethrough. Similar passagewayspass through the cathode 26 so that the oxidant for the fuel cell 2O mayflow therethrough. In the illustrated form of the fuel cell, thepassageways through the cathode are at a right angle to the passageways30, and are therefore out of view in FIG. 1. The passageways 30 connectat one end to a fuel input plenum (not shown) and at the other end to afuel output plenum (not shown). Similarly, the passageways in thecathode connect at one end to an oxidant input plenum (not shown) and atthe other end to an oxidant output plenum (not shown).

The pictured fuel cell structure is that illustrated in U.S. Pat. No.4,913,982. Other fuel cell structures such as that shown in U.S. Pat.No. 4,816,096 may be used in the fuel cell of the present invention, asmay other operable structures.

In one preferred form of the fuel cell 20, the anode 24 is a cermet ofcobalt or nickel metal and yttria-stabilized zirconia. The cathode 26 isstrontium-doped lanthanum manganite. The electrolyte 28 isyttria-stabilized zirconia. Other operable materials of construction maybe used.

An interconnect layer 32 is placed between the two electrochemical cells22 to connect them in series. In the conventional approach, theinterconnect layer 32 is a single layer made of magnesium-doped,strontium-doped, or calcium, cobalt-doped lanthanum chromite. Thepresent invention provides an improved construction and materialcomposition and a fuel cell utilizing these improvements.

A powder processing approach for preparing the fuel cell 20 is presentedin the '982 patent, and that same approach is operable with the presentinvention. Briefly, a mixture of the appropriate powdered materials witha binder and a plasticizer is prepared in a high-intensity mixer, foreach of the layers 24, 26, 28, and 32. Each of the mixtures is rolledbetween rollers into a tape. In one embodiment of this approach, threadsare incorporated into the anode tape and cathode tape to define thepassageways 30. The tapes are laminated together with other structure toform the fuel cell. The array is heated to an intermediate temperatureto extract the binder and plasticizer from each tape, and to burn outthe threads from the anode and cathode tapes. The array is furtherheated to a higher temperature to sinter the powders in each tapetogether and sinter the tapes and other structure together to form themonolithic solid oxide fuel cell. In another embodiment, the anode andcathode tapes are corrugated or embossed to provide the gas passageways.The tapes are bonded in proper sequence and orientation to produce agreen ceramic structure. The structure is fired to form the monolithicsolid oxide fuel cell.

The selection of the sintering temperature, time, and heating andcooling rates requires a careful balancing of the need to achieve astructure wherein each layer is sufficiently densified to perform itsfunction, and at the same time avoiding excessive loss of materialthrough volatilization and excessive interdiffusion of the layers. Theoptimal sintering temperature for the anode, cathode, and electrolytematerials is in the range of about 1400 C.-1500 C. At this temperature,however, the conventional doped lanthanum chromite interconnect may notachieve a sufficiently high density to prevent gas leakage betweenadjacent cells. To achieve such a high density in that material, it isnecessary to sinter at a temperature above 1600 C. But, at these highersintering temperatures, there may be volatilization and migration of thecathode material and interdiffusion of the electrode materials and theinterconnect. The chromium oxide of the interconnect itself can bevolatile at the higher sintering temperature, causing a change incomposition during sintering.

In one aspect of the invention, the interconnect layer 32 is made of amixture of an electrical conductor and a sinterable matrix thatdensifies in an oxidizing atmosphere at a temperature of less than about1500 C. In this approach, the interconnect layer 32 achieves sufficientelectrical conductivity for its purpose due to the presence of theelectrical conductor, which may be a metal (making the interconnectlayer 32 a cermet) such as a noble metal (e.g., platinum, palladium) ora ceramic such as a doped lanthanum chromite, a doped yttrium chromite,or a cobalt chromite. The selected electrical conductor must beelectrically conductive, stable during sintering, stable in both thefuel and oxidant environments, chemically compatible with the anode andcathode materials, and chemically compatible with the sinterable matrix.

The other component of the interconnect layer 32 is a sinterable matrix.The matrix is not required to be electrically conductive, but it must bestable during sintering, stable in both the fuel and oxidantenvironments, chemically compatible with the anode and cathodematerials, and chemically compatible with the electrical conductor. Thematrix readily sinters at the sintering temperature of the fuel cell,preferably at 1500° C. or lower temperature, and acts as a support thatholds the electrically conductive phase together. Possible matrixmaterials include zirconia, ceria, glasses, glass ceramics, and spinels.Thus, the final structure of the interconnect layer 32 is a composite ofan electrically conductive phase and a matrix binding phase.

The electrical conductor phase must have a volume fraction in themixture sufficient for the interconnect layer 32 to exhibit bulkelectrical conductivity. The precise volume fraction required willdepend upon the sizes and morphologies of the powders used tomanufacture the interconnect layer, but generally the volume fraction ofthe electrical conductor phase should be at least about 30 percent ofthe total volume of the interconnect layer 32.

In a variation of the composite interconnect approach, it has been foundthat a mixture of a doped lanthanum chromite-based ceramic and a mixtureof a doped yttrium chromite-based ceramic can be sintered at atemperature of from about 1350° C. to about 1500° C. in air to providethe required interconnect structure. This sintered structure has bothsufficient electrical conductivity and sufficient density, and has therequired compatibility with the fuel, oxidant, and other phases, asdiscussed above.

In this approach, yttrium chromite of formula Y_(l-x-y) Ca_(x) Zr_(y)Cr_(v-z) Zn_(z) O₃ or Y_(l-x') Ca_(x') CrO₃ was mixed with lanthanumchromite of formula La_(l-x") Ca_(x") Cr_(l-y") Co_(y") O₃. In theseformulations, x is from about 0.1 to about 0.3, y is from about 0.001 toabout 0.1, z is from about 0.1 to about 0.3, and v is from about 1 toabout 1.2; x' is from about 0.1 to about 0.3; and x" is from about 0.1to about 0.3, and y" is from about 0.1 to about 0.3.

The values of x, y, z, x', x", and y" are selected to provide desiredsinterability and electrical conductivity for the interconnect layer.For example, x, x', x", and y" should be in the range of about 0.1 toabout 0.3 to yield adequate electrical conductivity. Values more thanabout 0.3 tend to either decrease the electrical conductivity or formundesirable second phases. The values of y and z are chosen to enhancethe sinterability. If y or z is zero (i.e., zirconium or zinc is leftout), the as-sintered density of the interconnect falls significantly.Higher values tend to result in the formation of second phases. Thevalue of v is in the range of from about 1 to about 1.2 to providechromium nonstoichiometry in the material to improve sinterability.

A number of specimens were prepared to verify this approach anddetermine the optimum mixture of the modified yttrium chromite and themodified lanthanum chromite. In performing these studies, thestoichiometric values the variables were as follows: x=0.15; y=0.008,v=1; z=0.1; x'=0.2; x"=0.2, y"=0.1. The specimens were prepared by ballmilling the mixture of two powders and screened through a 100 meshscreen. Mixtures having 25% by weight, 50% by weight, and 75% by weightof the modified yttrium chromite, balance modified lanthanum chromite,were prepared. Bulk specimens were prepared by pressing. The sampleswere sintered at a temperature of 1400 C.

After sintering, the densities of the samples were measured. The highestdensities were obtained with those samples having 25% modified yttriumchromite, balance modified lanthanum chromite.

In a further aspect of the invention, it has been found that themodified yttrium chromite may be used by itself in the interconnectlayer 32, with a slightly higher sintering temperature than for themixture of modified yttrium chromite and modified lanthanum chromite, asjust described. Thus, the interconnect may be made of modified yttriumchromite of the form Y_(w-x-y) Ca_(x) Zr_(y) Cr_(v-z) Zn_(z) O₃, where wis from about 0.9 to about 1.1, x is from about 0.1 to about 0.3, y isfrom about 0.001 to about 0.1, z is from about 0.1 to about 0.3, and vis from about 1 to about 1.2. This material may be sintered to 90% oftheoretical density at a sintering temperature of 1500 C. in about 1hour.

As described above, the preferred values of w, x, y, v, and z wereselected to improve the conductivity and sinterability of the yttriumchromite. Higher values than indicated tend to lead to the formation ofsecond phases, and lower values tend to result in degradation ofelectrical conductivity or sinterability. For example, a material withw=1, x=0.15, y=0.008, v=1, and z=0.1 was found to densify when sinteredat 1500° C. for 1 hour. The material of the same formulation, exceptthat y=0, showed only slight densification.

In yet another aspect of the invention, illustrated in FIG. 2, aninterconnect means 34 has sublayers produced during the manufacturingoperation. The interconnect means 34 includes a first buffer layer 36having a region with a composition that is intermediate between that ofthe anode 24 and that of the interconnect layer 32. There is further asecond buffer layer 38 having a region with a composition that isintermediate between that of the cathode 26 and that of the interconnectlayer 32. The interconnect layer 32 is thus disposed between the firstbuffer layer 36 and the second buffer layer 38.

When interconnect layers of the modified lanthanum chromite type aresintered at temperatures above 1500° C., as may be required, there isthe possibility that the material of the anode 24 may interdiffuse withthat of the interconnect layer on one side of the interconnect layer,and that the material of the cathode 26 may interdiffuse with that ofthe interconnect layer 32 on the other side of the interconnect layer.If, on the other hand, sintering aids are added to the lanthanumchromite to lower its sintering temperature, ions from the sinteringaids may diffuse into the anode and/or the cathode, causing changes thatcan degrade their performance.

In the approach of FIG. 2, the buffer layers 36 and 38 are placed oneither side of the electrical interconnect layer 32, to form theinterconnect means. The buffer layer 36 has a composition that isintermediate between that of the interconnect layer 32 and that of theanode 24. The buffer layer 38 has a composition that is intermediatebetween that of the interconnect layer 32 and that of the cathode. Suchcompositions introduce no new elements into the structure which mightinterfere with the functioning of either the interconnect layer or theanode or cathode. The two buffer layers sinter at a temperature lessthan that of the interconnect layer 32, and therefore before theinterconnect layer during the sintering operation, forming a denserbarrier on either side of the interconnect layer during its sintering.The barrier prevents interdiffusion of species between the interconnectlayer and the layers on either side during sintering of the interconnectlayer, and reduces volatilization of the elements in the interconnectlayer by effectively sealing the surface of the interconnect layerduring sintering. The smaller compositional differences faced by thelayers also reduce the interdiffusional driving forces.

The buffer layers 36 and 38 may each be of a single composition, mayhave several compositions through the thickness of the layer, or mayhave a graded composition with a smoothly varying composition. Each ofthese structures may be made prepared with the tape casting technologydescribed in the '036 and '982 patents. For example, the buffer layer 36could be of a single composition of some intermediate value between theinterconnect layer and the anode, such as 50 percent interconnectmaterial and 50 percent anode material. It could instead have individualsublayers, such as a sublayer with 25 percent anode material and 75percent interconnect material adjacent to the interconnect layer, asublayer with 75 percent anode material and 25 percent interconnectmaterial adjacent to the anode layer, and an intermediate sublayer with50 percent anode material and 50 percent interconnect material betweenthe other two sublayers. Yet another possibility is a smoothly varyingcomposition between interconnect material adjacent to the interconnectlayer and anode material adjacent to the anode layer. The second bufferlayer can have similar types of structures, except formed of mixtures ofinterconnect layer material and cathode layer material.

Although particular embodiments of the invention have been described indetail for purposes of illustration, various modifications may be madewithout departing from the spirit and scope of the invention.Accordingly, the invention is not to be limited except as by theappended claims.

What is claimed is:
 1. A fuel cell, comprising:a pair of electrodes,includingan anode, and a cathode; a solid electrolyte disposed betweenthe anode and the cathode; and interconnect means for providing anelectrical connection located adjacent to one of the electrodes but notadjacent to the solid electrolyte, the interconnect means including aninterconnect layer having a composition selected from the group ofelectrically conducting materials consisting of(i) a mixture of anelectrical conductor and a matrix material that is sinterable in anoxidizing atmosphere at a temperature of less than about 1500 C., (ii) amixture of a lanthanum chromite-based ceramic and a yttriumchromite-based ceramic, and (iii) a yttrium chromite-based ceramic ofthe form Y_(w-x-y) Ca_(x) Zr_(y) Cr_(v-z) Zn_(z) O₃, where w is fromabout 0.9 to about 1.1, x is from about 0.1 to about 0.3, y is fromabout 0.001 to about 0.1, z is from about 0.1 to about 0.3, and v isfrom about 1 to about 1.2.
 2. The fuel cell of claim 1, wherein theinterconnect means includes,a first buffer layer having a region with acomposition that is intermediate between that of the anode and that ofthe interconnect layer, a second buffer layer having a region with acomposition that is intermediate between that of the cathode and that ofthe interconnect layer, and the interconnect layer disposed between thefirst buffer layer and the second buffer layer.
 3. The fuel cell ofclaim 1, wherein the anode, the cathode, the electrolyte, and theinterconnect means all have a flat layered morphology.
 4. The fuel cellof claim 1, wherein the electrical conductor is selected from the groupconsisting of a noble metal, doped lanthanum chromite, a yttriumchromite-based ceramic, and cobalt chromite.
 5. The fuel cell of claim1, wherein the matrix material is selected from the group consisting ofzirconium oxide, cerium oxide, a glass, a glass ceramic, and a spinel.6. A fuel cell, comprising:a pair of electrodes, includingan anodelayer, and a cathode layer; a solid electrolyte layer disposed betweenthe anode layer and the cathode layer; and interconnect means forproviding an electrical connection located adjacent to one of theelectrodes but not adjacent to the solid electrolyte, the interconnectmeans including an interconnect layer comprising a mixture of alanthanum chromite-based ceramic and a yttrium chromite-based ceramic.7. The fuel cell of claim 6, wherein the interconnect means includes,afirst buffer layer having a region with a composition that isintermediate between that of the anode and that of the interconnectlayer, a second buffer layer having a region with a composition that isintermediate between that of the cathode and that of the interconnectlayer, and the interconnect layer disposed between the first bufferlayer and the second buffer layer.
 8. A fuel cell, comprising:a pair ofelectrodes, includingan anode layer, and a cathode layer; a solidelectrolyte layer disposed between the anode layer and the cathodelayer; and interconnect means for providing an electrical connectionlocated adjacent to one of the electrodes but not adjacent to the solidelectrolyte, the interconnect means including an interconnect layercomprising a yttrium chromite-based ceramic of the form Y_(w-x-y) Ca_(x)Zr_(y) Cr_(v-z) Zn_(z) O₃, where w is from about 0.9 to about 1.1, x isfrom about 0.1 to about 0.3, y is from about 0.001 to about 0.1, z isfrom about 0.1 to about 0.3, and v is from about 1 to about 1.2.
 9. Thefuel cell of claim 8, wherein the interconnect means includes,a firstbuffer layer having a region with a composition that is intermediatebetween that of the anode and that of the interconnect layer, a secondbuffer layer having a region with a composition that is intermediatebetween that of the cathode and that of the interconnect layer, and theinterconnect layer disposed between the first buffer layer and thesecond buffer layer.